Possible Waveform Parameter Options for 6G

Apparently, the number of configurable parameters and numerologies for a single waveform may increase with 6G [75]. There may be more parameter variety than the 5G numerologies (e.g., flexibility in subcarrier spacings) [4]. Different lattice domains can be exploited with or without time-frequency. These domains pro-vide new types of numerology parameters (e.g., beamwidth parameter for space domain). Hence, variety of numerology parameters increases. In the future gener-ations, also different types of CP structures can be employed along with multiple

Figure 3.17: Different parameter assignments for each user in the same coverage area. It is assumed that coexistence of multiple waveforms in the same frame is also possible with multiple numerologies and additional waveform processing techniques.

numerologies [69]. Moreover, CP parameter can take more values independent from the subcarrier spacing. There can be different waveform processing meth-ods in 6G, so these methmeth-ods can bring new parameter types. Parametrization of new techniques will increase the number of waveform parameters. Addition-ally, multiple waveforms may be utilized together in the same frame for the next generation of wireless communications standards [76]. For example, it is possi-ble to use different waveforms together for beyond 52.6 GHz [1]. Coexistence of various standards may also trigger the designs of multiple waveforms in a frame.

Different waveforms can have specific numerologies with several types of param-eters. Therefore, there will be considerable amount of waveform parameters in 6G and options will exponentially increase with the number of waveform types.

An example usage of multiple waveforms and numerologies with some processing techniques is illustrated in Fig. 3.17. There is a necessity of configurable param-eter richness to meet the potential future requirements of 6G networks flexibly.

A projection of 5G NR is combined with potential waveform structures to

Figure 3.18: The list of numerology parameters and additional parameter types for CP-OFDM in 5G NR with example demonstrations.

forecast the waveform parameters in 6G. It is shown that there will be numerous waveform parameter options in the future. TPs will use all of these waveform pa-rameter options while assigning them to different users with the optimal decisions.

Possible differences between 5G and 6G waveform parameters are summarized in Table 3.8.

Multi-numerology based CP-OFDM waveform is standardized in 5G NR.

There are also optional waveform processing techniques like guard utilization, windowing and filtering in 5G NR as exemplified with Fig. 3.18. All of the op-tional waveform processing techniques have different type of parameters with various implementation structures. The number of numerologies and processing options will increase in 6G and the coexistence of multiple waveforms in the same frame may also be possible for 6G.

Table 3.8: Possible differences between 5G and 6G waveform parameters.

5G Waveform

Parameters

Possible 6G Waveform Parame-ters kHz and 120 kHz.

(3) Fixed CP ratio but two options for 60 kHz subcarrier spacing.

(1) There may be different lattice do-mains with or without time-frequency.

(2) Different domains provide new types of numerology parameters. (3) Common CP utilization [14] can be pre-ferred to make CP ratio flexible. (4) CP parameter can take more values independent from the subcarrier spac-ing. (5) There may be more parameter variety than the 5G numerologies [4].

(6) For the case of multiple waveforms, each waveform may have separate nu-merology options on the same or differ-ent lattice structures.

(1) Waveform processing methods need to be enhanced because of more INI effects. (2) For new lattice struc-tures, new INI management techniques need to be developed. (3) New type of interferences if multiple waveforms are utilized in the same frame. (4) New waveform processing techniques to control, reduce and exploit IWI. (5) Parametrization of each new technique will increase the number of waveform parameters.

New attempts to increase flexibility generally will increase the number of pa-rameter types in 6G. Some possible challenges can be listed as follow:

• There may be more numerology options in 6G but it will increase INI effects.

Hence, waveform processing techniques need to be enhanced. This situation will give rise the increment for the number of possible new parameters.

• New types of CP utilization methods can be preferred in 6G, such as com-mon CP [14]. It makes CP ratio more flexible but number of possible CP values increases compared to the numerology designs in 5G systems.

• If different lattice domains are used in 6G rather than the time-frequency, new types of waveform parameters will be included in the numerology sets.

Therefore, the number of numerology options will increase. Additionally, the current INI management techniques will be useless for different lattice domains. Then, new waveform processing techniques and the related pa-rameters need to be defined. This case will also increase the number of possible waveform parameters.

• Utilizing multiple waveforms in a single frame may be possible for 6G. Lat-tice structures can be different for each waveform as an important challenge.

Besides, there may be different types of numerology parameters for multi-ple waveforms. Additionally, IWI needs to be controlled by new waveform processing techniqes and the related parameters. Waveform parameters will increase in all of these cases for 6G.

3.2.6.1 Multiple Numerologies

In one of the first studies on multiple numerologies [20], channel-aware numerol-ogy assignments are done for multiple users with CP-OFDM. However, multi-numerology structure of 5G NR is flexible in order to consider different feedbacks including channel structures of users. Different frame parameters under four numerologies are provided for data transmission of 5G NR in Fig. 3.18 [14]. Nu-merologies can have various parameters that are dependent or independent of

each other. In 5G NR, there is only one main adjustable parameter which is a subcarrier spacing. The other parameters are generally dependent on it because of the practicality.

6G systems probably will come with more numerology structures that provides more flexibility. If the number of waveform related adjustable parameters increase with 6G, then there will be more options for multiple numerologies [4]. For example, adjustable CP duration and utilization are important concepts for 6G [69]. Using one common CP for different numerologies may be one of the new concepts in 6G and it changes the number of numerology options noteworthily [14].

Furthermore, possible implementation structures for multi-numerology CP-OFDM vary with different bandwidth part (BWP) operations in 5G NR [46].

BWP defines a fixed band with the same numerology. It is a bridge between numerologies and 5G NR scheduling. BWP operations are flexible, e.g., users with the same numerologies can be located contiguously in the frequency domain rather than creating several non-adjacent BWPs with the same numerology. Similarly, there are many different BWP implementation options or radio access network (RAN) slicing methods in 5G and beyond systems. The number of scheduling-related implementations and methods will likely increase in 6G.

3.2.6.2 Waveform Processing Techniques

Windowing usage, filtering usage, and inter-numerology guard utilization are ex-ample waveform processing techniques for cellular communications systems [77].

More waveform processing techniques can be developed in 6G to address prospec-tive requirements. Multiple numerologies and the other non-orthogonality sources increase the importance of waveform processing techniques [14]. These techniques require various adjustable parameters. For example, several prototype filters in the literature including rectangular, raised-cosine, Gaussian and so on, are pro-vided in [19]. There is a flexibility to apply windowing with different or same

roll-off factors on the subframes for each numerology or composite signal of mul-tiple numerologies at the transmitter. Receiver windowing is the another option.

Roll-off factor optimization is analyzed in [21]. Different filters, the related coeffi-cients, and roll-off factors increase the number of options for waveform processing techniques. Coexistence of 6G and the other standards will have even more op-tions, especially if there are multiple waveforms in the same frame.

3.2.6.3 Multiple Waveforms

In addition to multiple numerologies and waveform processing techniques, one of the future technologies is coexistence of multiple waveforms in the same frame together with multiple numerologies and processing techniques. In [78], frequency-domain non-orthogonal multiple access (NOMA) structure with two sets of orthogonal signal waveforms (CDMA and OFDMA) is presented. OFDM and OFDM-IM are used together as another NOMA scheme in [79] and [80].

Probably, there will be more studies on multi-waveform concept in 6G.

If there are multiple waveforms in the same coverage area [76], it means that there will be a tremendously high number of options for the waveform parameters.

Thus, the number of parameter options will double many times, depending on the number and types of waveforms. In addition, there will be more types of numerology structures if 6G waveforms use different lattice domains.

Chapter 4